This invention is related to wax-containing petroleum distillate having improved low temperature flow properties. More specifically, the present invention is related to a wax-containing middle distillate fuel oil having a boiling range within the limits of about 120.degree. and about 400.degree. C.
The problem of improving the cold flow properties of wax-containing distillates has become more pronounced recently because of increases in the demand for certain petroleum products, including kerosene and the middle distillates. Kerosene acts as a solvent for n-paraffin wax, and in former times was a significant component of middle distillate fuel oils. The increased demand for kerosene in jet fuels in more recent times has reduced the amount of kerosene available for use in middle distillate fuel oils. In addition, the increased demand for middle distillate fuel oils, particularly diesel fuel, has made it attractive to maximize the production of middle distillates.
The wax present in middle distillates precipitates at low temperature, forming large waxy crystals which tend to plug the small pore openings of fuel filters. This problem is particularly acute for diesel fuels, where the openings in the fuel filter typically are between about 5.0 and about 50 microns. Conventional pour depressants, which lower the pour point, i.e., the point at which the fuel can no longer be poured, may not be completely satisfactory for preventing pluggage of the fuel filters. While pour depressants often prevent the fuel from setting up as a gel, large wax crystals may be formed. However, to improve the cold flow properties of wax containing middle distillate fuels oils so that the wax does not plug the fuel filter pores, it is necessary that fine wax crystals be formed.
Considerable work has been directed at additives which improve the cold flow properties of the wax-containing middle distillate fuels. U.S. Pat. No. 3,790,359 is directed to the addition of from about 0.1 to about 3 weight percent of an essentially saturated hydrocarbon fraction substantially free of normal paraffinic hydrocarbons and having a number average molecular weight in the range of about 600 to about 3,000, in combination with a copolymer of ethylene and an unsaturated ester, where the copolymer has less than 6 methyl terminating side branches per 100 methylene groups. The weight ratio of the saturated hydrocarbon fraction to the copolymer is disclosed to range between about 25:1 to about 1:1.
U.S. Pat. No. 3,999,960 discloses the use of ethers, particularly alkyldiphenylether, to improve the cold flow properties of wax-containing middle distillate fuels.
U.S Pat. Nos. 3,444,082 and 3,846,093 teach various amides and salts of alkenyl succinc anhydride reacted with amines in combination with an ethylene copolymer as pour point depressants.
In addition, U.S. Pat. No. 4,755,189 discloses a middle distillate fuel having improved low temperature flow properties containing as a pour point depressant, a combination of an amorphous, normally solid, essentially saturated hydrocarbon fraction substantially free of normal paraffin hydrocarbons, a wax modifying random copolymer of ethylene and an unsaturated ester (vinyl acetate) and an alkylated diphenyl ether.
U.S. Pat. No. 4,211,534 discloses a middle distillate fuel additive comprising, as a pour point depressant, a combination of a copolymer of ethylene and an unsaturated ester, a second copolymer containing a polymerized C.sub.6 to C.sub.30 straight chain alkylester of a monoethylenically unsaturated carboxylic acid and an oil soluble nitrogen-containing compound selected from the class of long carbon chain amine salts and/or amides of hydrocarbyl carboxylic acids or anhydrides having 1 to 4 carbonyl groups.
While these and other additive combinations are useful in improving the cold flow properties of middle distillate fuels, frequently they do not reduce wax crystal size to the extent that would enable the waxy fuel to pass through the fuel filters at temperatures that are 5.degree. C. or more below the fuel cloud point.